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ABSTRACT Because juveniles of the fiddler crabs Uca pugilator and Uca pugnax have been difficult to distinguish morphologically, little comparative work has been done on young stages of these abundant and ecologically important inhabitants of salt marshes. Here I describe a way to differentiate early crab instars of the two species by the number of spoon-tipped setae on the meropodite of the second maxilliped. Several broods of each species were reared from eggs through the first several crab instars. By the time they reached 3-4 mm carapace width U. pugilator developed several spoon-tipped setae, whereas U. pugnax had none, indicating that juveniles of these species can be distinguished based on the early development of adult characteristics in U. pugilator. I also recorded molting frequency of juvenile U. pugilator through the sixth crab instar and found significant variability among broods in duration of several crab instars, a phenomenon that should be taken into account in future rearing studies.